President Barack Obama at the El Reno Federal Correctional Institution in El Reno, Okla. on July 16, 2015. Obama has commuted more prison sentences than any of the past seven presidents (other than Gerald Ford's massive commutation of convicted draft dodgers).

On Friday, President Obama, under Article II of the Constitution, commuted the sentences of 42 mostly low-level and nonviolent drug dealers – half of whom would have otherwise died in prison. It's the latest chapter – and perhaps the most visceral – in what has become a core White House mission: injecting US justice with mercy.

To be sure, Mr. Obama’s push for criminal justice reform in the wake of Ferguson, Mo., and other social-justice flashpoints come as one of precious few examples of bipartisanship in the Obama era. Many Republican governors and congressmen have joined a broader attempt to reduce the size of America’s incarceration complex, which houses half of the world’s prisoners.

But even Congressional moves toward criminal justice reform – most critically the 2010 Fair Sentencing Act that erased a 5-year-minimum sentence for simple crack possession – haven’t had the emotional influence of Obama’s commutation program, which has given hope to thousands of nonviolent federal prisoners who argue their sentences far exceed the seriousness of their crimes.

President Obama has now commuted a total of 348 people, more than the past seven presidents combined. (The count doesn’t include commutations under President Gerald Ford, who issued more than 10,000 commutations to convicted draft-dodgers in the wake of the Vietnam War.)

Much of the Obama administration’s work has been done courthouse by courthouse. For one, the Department of Justice has guided prosecutors to curb the use of mandatory minimums for drug crimes. But the president has also made broader strokes. In 2014, the Obama administration expanded the criteria for clemency-seekers, leading to record numbers of applications from prisoners. His staff says Obama is likely to announce more commutations before he leaves office.

A commutation is when a convicted criminal's remaining jail sentence is lifted. A pardon removes the sentence, the conviction, and any civil restrictions incurred by a felon (such as loss of voting rights).

In the end, however, Obama’s reform efforts are likely fall short of what some in his administration had hoped. Only a fraction of those potentially eligible for clemency will see it. The administration initially believed that as many as 10,000 prisoners would be eligible under the new 2014 guidelines, but that number has fallen to less than 2,000.

“The real question for President Obama is whether he grants commutations to all the people who meet his stated criteria,” New York University professor Rachel Barkow tells the Huffington Post. “And he’s short of that number right now, which should be more like 1,500 grants.”

No matter the eventual impact on his legacy, Obama’s commutations have proven restorative for many still suffering from what critics say is collateral damage caused by the War on Drugs.

That’s true for Teresa Mechell Griffin, who was sentenced as a drug trafficking conspirator in 1994 and given a life sentence for serving as her boyfriend’s "mule," or drug courier. She will be freed in October to go home to care for her disabled now-adult daughter.

“I know I did something wrong, but not enough to take away my life,” Ms. Griffin told the Huffington Post.

The personal stories of those freed early by Obama’s use of constitutional pardoning powers have likely served to further push public opinion toward deeper criminal justice reform.

But in other ways, Obama has been following, not leading, on justice reform. After all, four states now allow legal use of marijuana, part of a major shift in how at least parts of the country think about recreational drugs. And as part of the 2010 reforms, Congress changed a 100:1 sentencing disparity between crack and cocaine, by weight, to 18:1.

Such policy shifts by Congress and Obama also underscore the extent to which some imprisoned Americans feel their lives were ensnared in a cultural crisis.

"We were sentenced under the drug laws, and it was the war on drugs, so it's like [being] a POW in your own country," Ramona Brant, whose sentence was commuted by Obama last year, told CNN.

Released in February, Ms. Brant had served 21 years of a life sentence out of Charlotte, N.C. for helping her then-boyfriend sell cocaine. It was her first drug offense.